Is my Kill-A-Watt broken?

[Eli]

Ok, I have my variable power supply running an old radiator fan in the kitchen window. I wanted to see if it would use less power than a regular box fan.

With the fan being powered at 6V, my kill-a-watt says the power supply is drawing 25W from the wall. That's great! Much less power than a box fan, and probably even more CFM.

Now here's where it gets weird.

When I switch the Kill-a-watt to Amperage mode, it says the power supply is drawing 1.35A. Switching it back to Wattage, it says 25 watts.

/head explodes

1.35A at 120V is 150W+. Isn't it? ISN'T IT? Am I going crazy?

 

[ViRGE]

No, your Kill-A-Watt is right, your fan is just drawing the other 125W from subspace, so it is actually only using 25W of wall power.

 

[Eli]

Originally posted by: ViRGE
No, your Kill-A-Watt is right, your fan is just drawing the other 125W from subspace, so it is actually only using 25W of wall power.

Does it have something to do with the Power Factor?

The power supplies Power Factor is only 0.15.

Hmm...

1.35A @ 120V = 162W

162W * 0.15 = ~24-25W?

 

[mercanucaribe]

Power factor?

 

[AgaBoogaBoo]

Eli! AIM!

 

[imported_Kensai]

I would guess that there's some voltage conversion somewhere..

 

[PowerEngineer]

Originally posted by: Eli

Originally posted by: ViRGE
No, your Kill-A-Watt is right, your fan is just drawing the other 125W from subspace, so it is actually only using 25W of wall power.

Does it have something to do with the Power Factor?

The power supplies Power Factor is only 0.15.

Hmm...

1.35A @ 120V = 162W

162W * 0.15 = ~24-25W?

You are right about the power factor (which is the cosine of the angle between the voltage and current waves), but a power factor of 0.15 would be outrageously crappy. I don't know what this Kill-a-watt thing is, but maybe it's what is crappy.

P.S. -- You generally shouldn't expect better efficiency at lower volatges because it takes more current (and therefore more losses) to deliver the same power to move the air.

 

[Eli]

Originally posted by: PowerEngineer

Originally posted by: Eli

Originally posted by: ViRGE
No, your Kill-A-Watt is right, your fan is just drawing the other 125W from subspace, so it is actually only using 25W of wall power.

Does it have something to do with the Power Factor?

The power supplies Power Factor is only 0.15.

Hmm...

1.35A @ 120V = 162W

162W * 0.15 = ~24-25W?

You are right about the power factor (which is the cosine of the angle between the voltage and current waves), but a power factor of 0.15 would be outrageously crappy. I don't know what this Kill-a-watt thing is, but maybe it's what is crappy.

P.S. -- You generally shouldn't expect better efficiency at lower volatges because it takes more current (and therefore more losses) to deliver the same power to move the air.

The kill-a-watt is just a device that you plug in, that you can plug a device into. It will measure how much power the plugged in device is using. They're accurate to 0.2%, according to the manufacturer.

Link

The device plugged in is my HP Harrison 6427B variable DC power supply.

I'm not really sure what exactly you mean about not expecting better efficiency at lower voltages and stuff. Too many variables.

The box fans we've been using to cool off the house draw 75W on low, 95W on medium and 120W on high.

Someone gave me a radiator fan out of an 89 Camry, and it fit into my kitchen window. I wanted to see how much power it would draw, being powered by my power supply.

At 6V, it blows at least as much air as one of our 20" box fans on low, but it's only drawing 25W.. with a PF of 0.15. It's about as loud as a box fan on medium at this setting.

It shouldn't matter since residential customers don't pay for PF correction around here.

At 12V, it blows much more air than our box fans on high, but it's using only 115W from the wall. But it's also twice as loud as the box fans on high.

I'm going to put it on a PWM controller I have so I can vary it without messing with the power supply.

 

[eelw]

Whats the indicated VA and PF% on the meter?

 

[Eli]

Originally posted by: eelw
Whats the indicated VA and PF% on the meter?

PF is 0.15 and VA is 168.

Ooh.

So 168 * 0.15 = 25W?

 

[eelw]

Originally posted by: Eli

Originally posted by: eelw
Whats the indicated VA and PF% on the meter?

PF is 0.15 and VA is 168.

Ooh.

So 168 * 0.15 = 25W?

That's one damn inefficient power supply in your radiator fan though.

 

[Eli]

Originally posted by: eelw

Originally posted by: Eli

Originally posted by: eelw
Whats the indicated VA and PF% on the meter?

PF is 0.15 and VA is 168.

Ooh.

So 168 * 0.15 = 25W?

That's one damn inefficient power supply in your radiator fan though.

It's not in the radiator fan.. lol.

It's a bench supply, a HP Harrison 6427B.

0-20V, 0-15A.

It seems to only have such a low PF at these low voltages running the fan. At 12V, PF goes up to 0.30.. lol

At 4.5V, it's down to 0.11.

 

[imported_Kensai]

Originally posted by: eelw

Originally posted by: Eli

Originally posted by: eelw
Whats the indicated VA and PF% on the meter?

PF is 0.15 and VA is 168.

Ooh.

So 168 * 0.15 = 25W?

That's one damn inefficient power supply in your radiator fan though.

I agree with eelw.. That's worst than a Presshot.

 

[Eli]

Bump for people more knowledgable than I.. lol

 

[Mark R]

Wow. That's an impressively low power factor.

While you can marvel at that, take solace in the fact that you're probably only paying for the W (not the VA).

 

[0roo0roo]

supposedly the ac costcontrol is better

 

[Eli]

Originally posted by: Mark R
Wow. That's an impressively low power factor.

While you can marvel at that, take solace in the fact that you're probably only paying for the W (not the VA).

Yeah, pretty sure that only business customers have to pay the PF correction or whatever here in Oregon.

So wait, though....

Having a low power factor makes my power supply inefficient. I understand that.

But.... isn't this a good thing, from my point of view? Since I only have to pay for the watt hours?

 

[Mark R]

Having a low power factor makes my power supply inefficient. I understand that.

No. PF is seperate to efficiency.

Efficiency is how much energy you get out, divided by how much energy goes in.

Power factor is how many amps go in, divided by the minimum number of amps required for that power.

Your PSU is using 25 W - this means that 25 W of power has to be supplied by a power station somewhere.

However, it is using 1.35A which means that your house wiring, breakers, the electricity co's transformers and power lines have to carry that 1.35 (or equivalent). The electricity companies don't like this, because it means they have to oversize their transformers and power lines - which is why big industrial companies have to pay power factor penalties.

So, say you install 30 of these systems in your house. That would only be 750W, but you would still need to install 3 15A circuits to cope with the current.

 

[Eli]

Originally posted by: Mark R

Having a low power factor makes my power supply inefficient. I understand that.

No. PF is seperate to efficiency.

Efficiency is how much energy you get out, divided by how much energy goes in.

Power factor is how many amps go in, divided by the minimum number of amps required for that power.

Your PSU is using 25 W - this means that 25 W of power has to be supplied by a power station somewhere.

However, it is using 1.35A which means that your house wiring, breakers, the electricity co's transformers and power lines have to carry that 1.35 (or equivalent). The electricity companies don't like this, because it means they have to oversize their transformers and power lines - which is why big industrial companies have to pay power factor penalties.

So, say you install 30 of these systems in your house. That would only be 750W, but you would still need to install 3 15A circuits to cope with the current.

Hmmm....

So basically what you're saying is I should get a bunch of power supplies and inverters, and run everything in the house off those? Could save myself some money eh? LOL.....

 

[Analog]

Much of this has been said, but PF is the cosine of the difference in the phase angle between the current and voltage. A purely resistive load will have a PF of 1. In the power realm, you can envision a RIGHT triangle. Watts on one leg, Vars on the other leg (90 degrees to Watts), and then Volt-Amps on the hypotenuse. Watts being known as 'real power', Vars being reactive power.

Yes, you could probably scam power from the power company by doing this, but PG&E actually has a document that states they can charge you for using a lot of reactive power. You would generally have to use more than 400KW peak power though. Here is the document:

http://www.pge.com/docs/pdfs/biz/power_quality/power_quality_notes/economics.pdf

It is a good document on power, with many diagrams. I think it helps explain it better than just the text we have here. HTH

 

[Mark R]

PF is the cosine of the difference in the phase angle between the current and voltage.

That's an old fashioned definition.

A better definition is:
PF is the ratio of the magnitude of the in-phase component of the line current at the fundamental frequency, to the total line current.

So basically what you're saying is I should get a bunch of power supplies and inverters, and run everything in the house off those? Could save myself some money eh? LOL.....

LOL. Sadly, it won't save money - you'd still need the same number of Watts (which is what you pay for). However, the massive increase in the number of amps used, would mean having to upgrade greatly your wiring. So no benefit, no scam - just extra costs for you and your electricity supply company.

Low power factor doesn't give you more (or less) energy - it makes things look as if you are using more.

 

[Eli]

Originally posted by: Mark R

PF is the cosine of the difference in the phase angle between the current and voltage.

That's an old fashioned definition.

A better definition is:
PF is the ratio of the magnitude of the in-phase component of the line current at the fundamental frequency, to the total line current.

So basically what you're saying is I should get a bunch of power supplies and inverters, and run everything in the house off those? Could save myself some money eh? LOL.....

LOL. Sadly, it won't save money - you'd still need the same number of Watts (which is what you pay for). However, the massive increase in the number of amps used, would mean having to upgrade greatly your wiring. So no benefit, no scam - just extra costs for you and your electricity supply company.

Low power factor doesn't give you more (or less) energy - it makes things look as if you are using more.

Ahh.. so it really wouldn't work that way? Hmm..

Well why not?

If I ran things that had a PF of 1.0 off of a power supply that had a PF lower than 1.0, wouldn't I effectivly be saving money?

 

[Mark R]

If I ran things that had a PF of 1.0 off of a power supply that had a PF lower than 1.0, wouldn't I effectivly be saving money?

Let's say you have a 100 W, 100 V lamp - it has a PF of 1.0 - so at 100V it needs 1.0 A.

Now for the power supply: You need to get 100 W out of it. You can't get 100 W without putting 100 W in. So it would need 100W input. Because you still need 100 W of power, you pay the same amount.

However, your PSU would need 2 A at 100 V. You might not be charged for the extra amps, but they will put extra stress on your wiring.

 

[Eli]

Originally posted by: Mark R

If I ran things that had a PF of 1.0 off of a power supply that had a PF lower than 1.0, wouldn't I effectivly be saving money?

Let's say you have a 100 W, 100 V lamp - it has a PF of 1.0 - so at 100V it needs 1.0 A.

Now for the power supply: You need to get 100 W out of it. You can't get 100 W without putting 100 W in. So it would need 100W input. Because you still need 100 W of power, you pay the same amount.

However, your PSU would need 2 A at 100 V. You might not be charged for the extra amps, but they will put extra stress on your wiring.

Ahh, I see. That makes more sense.

Although, I'm going to have to test this for myself. lol

I've got a 100W aquarium heater here.

It draws 97W, PF is 0.99.

I'm going to run it for an hour plugged into the wall.. It should draw about 1.0kWh.

And then I'll run it from my power supply hooked up to a 700W inverter for an hour, and we'll see if it has also drawn 1.0kWh.

What a great way to waste 2 hours. Staring at watt meters.

 

[sharkeeper]

This is typical when your measuring apparatus is not designed for non linear loads.

You need a Dranetz for that. Just a little more $$$ than your KAW!